Decoherence in Josephson Qubits from Dielectric Loss

TL;DR

This paper identifies dielectric loss from two-level states as a key decoherence source in superconducting qubits and demonstrates that using better dielectrics and smaller junctions can significantly improve coherence times.

Contribution

It provides a detailed model of dielectric loss from two-level defects and shows how improved materials and design reduce decoherence in Josephson qubits.

Findings

Dielectric loss from two-level states dominates decoherence.

Using low-loss dielectrics and small junctions improves coherence.

Energy relaxation rate improved by a factor of 20 with redesigned qubit.

Abstract

Dielectric loss from two-level states is shown to be a dominant decoherence source in superconducting quantum bits. Depending on the qubit design, dielectric loss from insulating materials or the tunnel junction can lead to short coherence times. We show that a variety of microwave and qubit measurements are well modeled by loss from resonant absorption of two-level defects. Our results demonstrate that this loss can be significantly reduced by using better dielectrics and fabricating junctions of small area $\lesssim 10 \mu \textrm{m}^2$. With a redesigned phase qubit employing low-loss dielectrics, the energy relaxation rate has been improved by a factor of 20, opening up the possibility of multi-qubit gates and algorithms.